It makes sense for her to convert. Maybe just for those two it wouldn't be important, but as your friend said, for their kids, it's less messy if both parents are the same thing.

Since it's pretty much a "conversion in name only" (i.e. neither is practicing their religion) it has no real spiritual meaning aside from the two being unified under one banner. Also they can marry in a church which makes things easier.

Probably just a political decision to avoid future problems, and one that makes perfect sense.